In such a board, the multi-layer structure is ordinarily supported by a substrate made of an insulating slab that incorporates the distribution planes of the supply voltages of the integrated circuit or circuits. The substrate may be a slab of co-fired ceramic, or a slab made of some organic material, such as plastic. However, the board may be simply formed of the multi-layer structure, taking the form of a flexible board.
Alternatively, the board may be a slab of semiconductor material incorporating integrated circuits and covered with a multi-layer structure for interconnection of these integrated circuits by the WSI (wafer scale integration) technique. In all these boards, the multi-layer structure comprises a stack of alternating conductive and insulating layers. In this stack, an insulating layer is pierced with via-holes for electrical connection of the adjacent conductive layers. The conductive layer is ordinarily made of aluminum or copper, and at present, the insulating layer is preferably made of a polymerized material such as polyimide.
One problem in manufacturing the multi-layer structure is how to obtain relatively planar layers. One conventional manufacturing method comprises forming the conductors of a conductive layer and covering the conductive layer with an insulating layer of polymerizable material in the form of viscous liquid, known in this field as paste. The paste has the advantage of having a surface area with steps or lower levels that are not as high as the corresponding edges of the conductors underneath. This method may comprise spreading a drop of polymerizable material by centrifugal force, or spray application of the paste, or making a coating of the paste, for example by serigraphy. The paste is then polymerized. Naturally this method is used under such conditions as to lend the polymerized layer the great thickness required to assure the desired insulating between two conductive layers one on top of the other. The insulating layer is covered with a mask that defines the location of the via-holes to be formed. In a conventional embodiment, the via-holes are flared, and the conductors of the upper conductive layer extend over both the insulating layer and the walls of the via-holes that have just been formed, as well as over the regions of the conductors making up the bottom of the via-holes. The result is an upper conductive layer with numerous depressions at the level of the via-holes. Understandably, stacking numerous layers accentuates the depth of the depressions and consequently entails the risk of creating discontinuities in the conductive layers deposited. Moreover, the formation and flaring of the via-holes are delicate steps in the method.
One improvement to this method comprises avoiding the superposition of via-holes by offsetting them in a spiral about a vertical line. A first disadvantage of this is that it reduces the density of the via-holes in the same insulating layer and consequently reduces the overall density of conductors in the multi-layer structure. A second advantage is that although the staggered disposition of the via-holes is quite fast, it limits the number of reliable layers in the structure. Disposing them in a spiral 90.degree. from one another, for example, means that a via-hole in the fifth layer is superimposed on a via-hole of the first layer and undergoes the deformations occasioned by the four via-holes in between. In practice, this method is ordinarily limited to the deposition of about five superimposed conductive layers.
A more recent solution to the problem has been to form via studs on the conductors of a conductive layer and to cover all of this with a paste that is polymerized to obtain the insulating layer. The insulating layer has the great thickness required to assure the desired insulation between two superimposed conductive layers and covers the via studs with a lesser thickness of insulating material. The insulating layer is covered with a mask that has the configuration of the via-holes. Next, a selective attack of the insulating layer is performed to uncover the upper surfaces of the via studs and to flare the via-holes. Because of the via studs, these via-holes are markedly less deep than those obtained by the method described in the previous paragraph. The upper conductive layer is accordingly relatively more flattened. Thus this method has the advantage of stacking a greater number of reliable conductive layers in the multi-layer structure.